​Let me define stock photography by giving an example. An advertising agency is creating a magazine advertisement for a client. The agency needs a photo of a woman standing in a park, wearing a funny hat. The agency can get this photo in one of two ways:

The first option would be for the agency to contract with a photographer, hire a model, purchase a whole bunch of hats, scout around for just the right park, pay for the necessary permits, etc, then take the pictures.

The other option, usually quicker and much cheaper, would be for the agency to locate an already existing photo of a woman standing in a park, wearing a funny hat. For that, the advertising agency would contact a type of business called a stock photo house.

Stock photo houses hold thousands of photos, supplied to them by many shooters (who are referred to as stock photographers). In this case, the agency would go to a stock house's website and search online for the needed image. When found, that image would be sent to the agency for them to use in their magazine advertisement.